A rectifier is known to convert an alternating voltage or current signal into a direct voltage or current signal. It is used in a large field of applications and industrial processes.
The controlling of an output voltage of a rectifier is usually done by the following approach: For coarse regulation of the output voltage of the rectifier, a transformer with on-load tap changers (OLTC) is used. For the fine regulation of the output voltage, saturable reactors are used which are coupled to the rectifier for controlling the rectifier. It is common practice in prior art that the controlling of a reactor is implemented by two current circuits: One bias circuit and one control circuit. The current of the bias circuit may be implemented by a one phase variac and a B2 diode rectifier. Alternatively, a transformer with a number of fixed taps on its secondary side and a rectifier circuit may be used. The current of the control circuit may be implemented by a B6 thyristor rectifier. The control circuit usually consists of a controllable direct current source that feeds the control winding of a saturable reactor. Alternatively, the control current may also be a variable alternating current signal, which is rectified by a passive rectifier, for example using diodes.
However, the disadvantage of such an implementation is that for each of these current circuits two separate rectifier bridges are needed—one bridge for the bias current circuit and one bridge for the control current circuit. However, this leads to more complexity and thus to an increase of manufacturing and maintenance cost for the controlling of a rectifier. A further disadvantage is that the variac used for the bias circuit has to be set up manually leading also to an increased amount of service and maintenance time when commissioning and operating such a rectifier system.
In particular during commissioning of the rectifier and in case of changes of process parameters during operating of the rectifier, the bias current and the control current range need to be adapted to allow for an optimum control response of the rectifier system. This adjustment is usually done manually to find optimum values for a suitable linear operating range in the characteristic hysteresis curve of the controllable reactor element. However, this is often time-consuming when there is a need to react promptly due to changes of process parameters. The rectifier must then adopted to the new process parameters to allow for an optimum control response of the technical system coupled to the rectifier.
An objective of the present invention is to provide an improved control system for controlling a rectifier in order to make it simpler and faster to configure and to adopt the operating of a rectifier. In particular, in cases when the operating conditions of the rectifier are changing the operating behavior of the rectifier should be adopted accordingly in order to be able to operate the rectifier in an optimal operating range. Another technical object of the present invention is to provide an improved control system for controlling a rectifier reducing the amount of manufacturing and maintenance costs. A further technical object of the present invention is to provide for an improved method for controlling a rectifier in order to reduce the time for adjustment of the rectifier in case of changes of process parameters and as a result providing an optimum control response for the technical system that may be coupled to the rectifier.